Overview
This lecture explains the nitrogen cycle, its key steps, reservoirs, and how it differs from the carbon cycle, focusing on human impacts and critical chemical transformations.
Nitrogen Cycle Basics
- The nitrogen cycle is the movement of nitrogen between sources, sinks, and reservoirs.
- Nitrogen reservoirs hold nitrogen for shorter periods than carbon reservoirs.
- Main nitrogen reservoirs include plants, soil, and the atmosphere (which contains 78% nitrogen as N₂ gas).
- Atmospheric N₂ is not usable by most plants and animals.
Importance of Nitrogen
- Nitrogen is essential for making DNA and amino acids in living organisms.
- Most organisms cannot use atmospheric N₂ directly.
Key Steps in the Nitrogen Cycle
- Nitrogen Fixation: Conversion of atmospheric N₂ into usable ammonia (NH₃) or nitrate (NO₃⁻) mainly by bacteria or lightning.
- Nitrogen-fixing bacteria exist either free in soil or in symbiotic relationships with legumes (bean and pea family).
- Synthetic Fixation: Humans fix nitrogen into nitrates for fertilizers via industrial processes.
- Assimilation: Plants absorb nitrates or ammonia; animals obtain nitrogen by eating plants or other animals.
- Ammonification: Decomposers and soil bacteria convert waste and dead biomass back into ammonia.
- Nitrification: Bacteria convert ammonium (NH₄⁺) into nitrite (NO₂⁻) then nitrate (NO₃⁻).
- Denitrification: Bacteria convert nitrate (NO₃⁻) in the soil back into gaseous forms (mainly N₂O, then N₂), returning nitrogen to the atmosphere.
Human Impacts on the Nitrogen Cycle
- Agricultural soils emit nitrous oxide (N₂O), a greenhouse gas, especially with over-irrigation.
- Excess fertilizer use leads to ammonia volatilization, causing acid rain and human respiratory issues.
- Leaching from over-fertilization results in nitrate runoff, leading to water eutrophication and plant death in aquatic systems.
Key Terms & Definitions
- Nitrogen Fixation — Conversion of atmospheric N₂ into biologically usable forms (NH₃, NO₃⁻).
- Assimilation — Uptake of nitrogen by plants/animals to build DNA and proteins.
- Ammonification — Decomposition of organic matter, returning nitrogen to the soil as ammonia.
- Nitrification — Bacterial conversion of ammonium into nitrite and then nitrate.
- Denitrification — Process where soil bacteria convert nitrate into nitrogen gases (N₂O, N₂), returning it to the atmosphere.
- Eutrophication — Overgrowth of algae in water due to excess nutrients, leading to oxygen depletion.
Action Items / Next Steps
- Describe one chemical transformation in the natural nitrogen cycle and explain its importance to an ecosystem.